1. Field of the Invention
The present invention concerns a method and a device in order to control the workflow of an MR measurement which is implemented with continuous table feed. The MR signals are in particular detected with “single shot” sequences.
2. Description of the Prior Art
MR measurements with a table driven continuously through the magnetic resonance system serve to extend the field of view (FOV) in the direction of the table feed (FOVz) or a movement path of the table, and simultaneously to limit the measurement region within the magnetic resonance system. The technique competing with continuous table feed is the acquisition of the extended FOV in multiple respective stations with a stationary table. After all data of a station have been acquired, a patient is thereby driven with the table to the next station, wherein the MR measurement is interrupted during the travel. A detailed overview of known techniques with continuous table feed is found in “Principles of Whole-Body Continuously-Moving-Table MRI”, Peter Börnert and Bernd Aldefeld, Journal of Magnetic Resonance Imaging 28:1-12 (2008).
The most important techniques with a continuous table feed can be subdivided roughly into two-dimensional axial MR measurements with a table feed perpendicular to the image plane and three-dimensional techniques in which the readout direction is oriented parallel to the direction of the table feed. The present invention is concerned in particular with the two-dimensional axial MR measurements with a table feed perpendicular to the image plane.
An optimal optimization of a method for two-dimensional axial MR measurement with a table feed perpendicular to the image plane depends on a sequence type that is used for the MR measurement. Differentiation is made between sequences known as “single shot” sequences and known as “multi shot” sequences.
“Single shot” sequences include sequences in which all k-space lines of an image or a slice are read out after a single radio-frequency excitation pulse (for example “echoplanar imaging” (EPI) sequences or “half Fourier single shot turbo spin echo” (HASTE) sequences), as well as sequences with short repetition time in which multiple excitation pulses are switched per slice and the data of a slice are completely acquired before the acquisition of the data of the next slice is begun (for example TrueFISP or TurboFLASH).
Sequences with a longer repetition time in which multiple excitation pulses are switched per slice and different slices are excited and read out during a repetition interval belong are the “multi shot” sequences.
The present invention concerns “single shot” sequences.
The simplest technique with continuous table feed during the measurement using a “single shot” sequence is to acquire data continuously in the center of the magnetic resonance system while a patient or an examination subject is driven through the magnetic resonance system with continuous speed. If multiple radio-frequency pulses are generated per slice, in an optimal implementation of the MR measurement the location or the position of the excitation or the inversion is adapted to the table speed such that (assuming a rigid examination subject) the same slice in the examination subject is radiated by the radio-frequency pulses. This technique, known as the single slice technique, has the advantage that all slices within the volume segment that is to be examined are similarly acquired in the center of the magnetic resonance system, at the location of the lowest distortion.
In the use of fast sequence techniques in which the acquisition time per slice is short relative to a typical time scale of a physiological movement within the examination subject, this physiological movement is essentially “frozen” during the acquisition of a slice, as is the case with a stationary patient bed in an MR measurement.
In this technique the maximum velocity vmax of the patient bed is established by the following Equation (1):
                              v          max                =                  d          TA                                    (        1        )            
TA is the acquisition time per slice and d is the distance in the direction of the table feed between the center lines of the excitation profiles of two adjacent slices.
Slower table velocities than vmax can also be realized in that the data acquisition between two measurements is interrupted for a pause time TP. The repetition time between the acquisition of successive slices is then defined by the sum of acquisition time TA and pause time TP, wherein the table velocity v that is then used can be calculated via the following Equation (2):
                    v        =                  d                      TA            +            TP                                              (        2        )            
In clinical imaging a typical slice interval d is between 4 and 8 mm. In the “single shot” sequences relevant to the present invention, the resolution in the slice (“inplane”) that is normally required can be realized with an acquisition time of less than 1 s per slice. Given a repetition time of 1 s, a table velocity between 4 and 8 mm/second can thus be realized with what is known as the single slice technique. Such a table velocity is sufficient for many applications. However, such a table velocity leads to unwanted interactions between spatially adjacent slices which are acquired in chronological succession in the technique proposed above. The most important causes of the unwanted interactions are the following:
Crosstalk                Every selective radio-frequency pulse has an imperfect excitation profile due to its finite duration. Technically, every radio-frequency pulse thereby unavoidably also affects regions which are located outside of a slice to be excited by the radio-frequency pulse. This effect occurs most strongly between immediately adjacent slices. If the data of one slice are acquired before the magnetization disturbed by a radio-frequency pulse for the neighboring slice has returned to its equilibrium state (which is the case after approximately four to five times the T1 time of the tissue of the slice), the signal intensity (and therefore the signal/noise ratio) thereby decreases and the contrast of the image is altered.        
Preparation Pulses                Sometimes selective preparation pulses are generated before the first excitation pulse of a slice. These are thereby radio-frequency pulses which normally invert or saturate the slice and serve either to suppress an unwanted tissue type (for example fat, blood, CSF (cerebrospinal fluid)) in the image or to intensify a contrast between different tissue components in the image. The preparation pulse is thereby normally selected to be wider than the slice thickness either in order to achieve a uniform inversion or, respectively, saturation of the slice to be measured given an acceptable duration of the preparation and given an acceptable SAR (“Specific Absorption Rate”) exposure, or—given a fluid (for example blood or CSF) in order to ensure that fresh fluid quantities flowing into the slice from neighboring slices in the time between the preparation and the excitation are inverted or, respectively, saturated. Given the use of such preparation pulses, a sufficiently long wait time between the acquisitions of spatially adjacent slices is thus extremely important since the use of preparation pulses before a decay of a magnetization via preparation pulses for a neighboring slice can completely suppress or even reverse the desired effect.        
Consideration of Breathing                Even if the acquisition time per slice is short relative to a typical time scale of human breathing, the case can arise that the organs or volume segments examined by the MR measurement move counter to the direction of the table feed due to the breathing. In this case it is possible that data of a tissue are acquired before a magnetization that was disrupted in this tissue due to a preceding measurement has returned to its equilibrium state, which can lead to a significant signal loss.        
The disadvantageous interactions described in the preceding can be avoided in the single slice technique via a sufficiently long repetition time. However, the table velocity (see Equation (2)) is therefore also reduced, which disadvantageously leads to a lengthening of the total measurement time. The total measurement time Ttotal can be calculated with the following Equation (3) from the extent of the examination region in the direction of the table feed (FOVz) depending on the table velocity v.
                              T          Total                =                              FOV            z                    v                                    (        3        )            
The total measurement time is in turn directly linked with the costs of an examination by means of a mews system, which is why a lengthening of the repetition time normally does not represent an acceptable solution.